The moving source emits fS wavefronts per second. The distance between the wavefronts is
in the
side which the source is moving from. Thus the stationary detector intercepts
fS wavefronts per second.

D

S

fD >fS

The moving source emits fS wavefronts per second. The distance between
the wavefronts is
in the side which the source is moving to. Thus the stationary detector intercepts
fSwavefronts per second.

The source emits fS
wavefronts per second. When the source and detector are both
stationery, the detector intercepts fS wavefronts per second and
hence the frequency measured by the detector fD is the same
as the source frequency fS.

2-7 The Doppler effect

Moving source

The
speed of the source vS and the speed of the detector vD are measured relative to air. If air
is stationary, these will be the same as that measured relative to ground.

→

vS

Click and drag the blue line to change the frequency of the sound source